[Test-Equipment] question on Polarad rms/dbm

[eugene at hertzmail.com]

Hi all again,

I am working on learning about measuring and rms/Vp-p/dbm and I really
think I have it down! However, I have one piece of test equip that has
me scratching my head (aside from my 410B!).

I have been comparing the usage of two generators I have, one an HP
8657A recently calibrated. The other a Polarad SPN 1Hz-1.3MHz sine wave
generator.

Both have 50 ohm output (the SPN has 50 ohm,600 ohm and 5 ohm output
selectable, I have selected 50 ohm).  I am connecting each to my scope
using a tektronix passthru 50 ohm terminator.

The HP & scope combination seem to function exactly as I would expect,
meaning if I key the amplitude as 0 dbm and switch to V display the
display shows 224mv (I am pretty sure this means 224Vrms). And, when I
display this on my scope, I get 680mVp-p (/2 * .707) gives 240mV (pretty
close).

On the Polarad, things are strange. When I set its amplitude to 0 dbm,
the scope indicates also 680mVp-p (good) but when I change its display
from dbm to mV it indicates 447mV?! Switching back to dbm shows 0, but
mV is 447.

What is the Polarad indicating? Seems its displaying 2*Vrms ? Is this
some alternate standard of displaying amplitude? Could this be the
correct Vrms for a different output impedance other than 50 ohms?

I guess I'm trying to figure out if a) this is working correctly by
showing 2*Vrms when in mV display - I just don't understand why its
indicating this, or b) if this unit has a problem with it.

Can anyone tell me what's going on here?
Thanks
Eugene 
PS anyone have a manual on the SPN? Or alternately, can anyone tell me
where to find the frequency adjustment? I'd like to correct the
frequency just a bit. Thanks!